Microsoft has years of experience rolling out Windows software updates to computers made by HP and Dell, but devices built by phone manufacturers and sold by cellular carriers turn out to have some key differences, Belfiore said.

"We found issues with the way the update was getting deployed on phones because of things we hadn't anticipated," Belfiore said. "Phones are different in some significant ways that I think we've now got a handle on."

With smartphones, OEM vendors play more of a role in the code used in the core operating system than PC manufacturers do, he said. This results in manufacturing issues that are different on phones than PCs.

"We have tried to improve the team that does this infrastructure," Belfiore said. "We've added people, we've staffed it up to make sure we do it as quickly as we can... We expect that we're going to get these problems licked and get good at this and have no issues in the future."

Several weeks ago, Belfiore stated publicly that most phone users had gotten the February update, and "I was wrong," he said at MIX11.

Updates were tested on "pre-manufactured phones," but when they rolled out to phones in the hands of users some issues cropped up. A small group of phones from one vendor - less than 100 - contained a setting to indicate when something goes wrong. The phones worked fine at first, but once an update was pushed out to users the phone "switched back into factory-diagnostic mode," Belfiore said.

This was one of several problems Microsoft encountered. "There was no one issue, and none of the issues were particularly widespread," he said.

Once Microsoft ran into problems it decided to halt the update process to ensure its reliability. Some users got their updates later than others, but Belfiore said that is preferable to making everyone wait.

Google's Android goes through a similar process, with carriers and phone manufacturers pushing out updates at different times to users of different phones. But Android's updates have also gone a lot smoother than Microsoft's.

Belfiore said Microsoft had promised to add copy-and-paste functionality to Windows Phone 7 earlier this year, but that was delayed. "It's now rolling out," he said. "It's not available to everyone yet because some mobile operators are still testing it."

Microsoft's Windows Phone 7 is well behind Android and Apple's iPhone in market share. Belfiore delivered his explanation of the Windows Phone7 update process at the beginning of his keynote, and was expected to focus on future improvements to WP7 for the rest of the session.

"We're pretty optimistic that we've gone thru that learning process and we're not going to face those issues in the future," he said.